As the issue of environmental protection and pollution is becoming more severe, alternative energy sources are being developed around the world as a solution. As a secondary battery in a field of endeavor focused on alternative energy development, a small-sized lithium secondary battery has been replaced by a lithium secondary battery as a power source for operating mobile electronic communication equipments that require high performance, such as, for example, camcorders, mobile phones, laptop computers, and the like, and occupies a dominant position as a power supply. Recently, development is being actively made on a medium and large-sized lithium secondary battery in a hybrid electric vehicle (HEV) and an electric vehicle (EV) using high output characteristics. Further, research and development is being actively made in various application fields of industries all over the world including Japan, Europe, USA as well as Korea to develop a lithium secondary battery as an environment-friendly power source of uninterruptible power supplies, electromotive tools, ships, satellites, radio sets and weapon systems for military purposes, and the like.
A lithium secondary battery has a structure comprising an assembly including a cathode and an anode coated with an active material capable of intercalating or disintercalating lithium ions and a porous separator membrane interposed between the cathode and the anode to electrically separate the cathode from the anode, and an organic electrolyte solution or a polymer electrolyte solution including lithium salts, filled in the assembly. A lithium metal oxide having a high average voltage is used as a cathode active material, for example, LiCoO2, LiNiO2, LiNixCoyAlzO2, LiNixCoyMnzO2, LiMn2O4, and the like, a carbon material or a metal or non-metal oxide having a low average potential is used as an anode active material, and a porous sheet made using a polyolefin-based polymer, for example, polyethylene (PE), polypropylene (PP), and the like, is mainly used as a separator membrane.
However, in a case in which the cathode active material discussed in the foregoing is used, a surface transition metal-deficient layer caused by a decomposition phenomenon of the electrolyte solution is formed, and obstructs movement of the lithium ions and electrons, affecting high rate discharge, which causes gas to generate inside of the battery due to a side reaction with the electrolyte solution, thereby releasing metal, so that cycling characteristics deteriorate due to a structural change, and further, oxygen is generated due to an increase in internal temperature of the battery caused by an abnormal operation of the battery, which involves a risk of thermal runaway, raising safety concerns.
In a case in which the carbon-based anode active material is used, the lithium ions inserted in the layered structure exhibit an irreversible capacity of 5 to 25% during initial charge and discharge, and this irreversible capacity leads to consumption of the lithium ions and prevents complete charge or discharge of at least one active material, resulting in reduced energy density of the battery. Also, the decomposition reaction of the electrolyte solution on the surface of the active material forms a passivation layer or a solid electrolyte interface (SEI) on the surface of the active material, and when the passivation layer is non-uniform or excessively thick, the resistance increases, causing deterioration in high rate characteristics. Further, due to a lithium compound generating on the surface of the anode, a capacity reduction and output characteristics degradation results from lithium loss, and in the long run, deterioration in cycle characteristics occurs.
The polyolefin-based separator membrane has a safety melting point of 200° C. or below, and so in terms of safety, it is inevitable to use a porous separator membrane having a shut-down function. Also, if temperature continues to increase after shut-down, shape maintenance is an important requisite. However, if an overcurrent flows due to a short circuit caused by an internal or external factor, generally, the olefin-based separator membrane experiences thermal contraction and melting due to heat generation, circuit malfunction, or external temperature increase, causing a short circuit between electrodes, which may produce fire.